Positron Annihilation Lifetime Spectroscopy (PALS) is a technique for direct measurement of sub-nanometer sized molecular free volumes. General operations in the PALS technique includes emitting positrons from an inspection region in a material being tested and then measuring the length of time until it annihilates with one of the material's electrons, producing gamma rays
Positrons are antiparticles of electrons. A positron collision with an electron results in the annihilation of both particles and an emission of two characteristic 511 keV gamma rays. The lifetime of positrons is a measure of the local electron density at the point of annihilation. The annihilation can be detected by virtue of the gamma rays emitted. Positron lifetime techniques are among the few methods that are sensitive to voids on the mono-atomic scale.
Spatial resolution of the PET tomograms is limited by positron range. Traveling of the positrons in human tissue before undergoing annihilation may lead to a positional inaccuracy, which reduce a PET image quality. One technique directed to the positron range issue applies a static axial magnetic field on the positrons. When positrons are in the axial magnetic field, they experience a Lorentz force. Since the Lorentz force is perpendicular to the applied magnetic field direction, the positrons may freely fly along the magnetic field direction while being constrained in all planes whose normal vector is in the applied magnetic field direction.
One shortcoming of the axial magnetic field technique is that the positron range may be confined only in the transaxial plane, i.e., plane(s) transverse to the direction of the magnetic field. Costs of the axial magnetic field technique therefore include a loss in the axial resolution (along the magnetic field direction). The loss of axial resolution can produce a shine-through artifact, which degrades image quality. One additional shortcoming of the axial magnetic field technique is that a high strength (up to 10 T) is required for the applied magnetic field to effectively confine the positron range in the transaxial plane.
There is, therefore, a need in the art for a method to confine the annihilation rage of positrons in all three dimensions in PET imaging systems. There is also a need in the art for a confining of the positron annihilation range, without requiring high-strength magnetic fields.